Refrigeration system

ABSTRACT

A refrigeration system comprises a mixing tank for a slurry of solid particles in a liquid, said mixing tank having first and second inlets and an outlet. A sublimator has a bottom inlet, a top outlet and several internal paths connecting the inlet and the outlet, said internal paths having no descending parts. A first conduit connects the outlet of the mixing tank to the bottom inlet of the sublimator via a pump, there being no descending parts between the pump and the inlet of the sublimator. A separator has an inlet and top and bottom outlets. A second conduit connects the outlet of the sublimator to the inlet of the separator, the bottom outlet of the separator being connected to the first inlet of the mixing tank. A compressor has an inlet and an outlet, and conduits connect the top outlet of the mixing tank to the inlet of the compressor and the outlet of the compressor to the second inlet of the mixing tank.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a refrigeration system using a slurryof solid particles in a liquid as a cooling medium. The particles shouldbe substantially immiscible in the liquid and sublimate at thetemperatures and pressures used in a sublimator (evaporator) of therefrigeration system.

2. Background of the invention

DE-A-30 04 114 describes a refrigeration system using particles of solidcarbon dioxide and terpene as transport liquid. More particularly,liquid carbon dioxide (carbonic acid anhydride) is expanded below thetriple point such that it converts to carbon dioxide particles (snow)and vapor. The carbon dioxide particles are mixed with terpene and theresulting slurry is pumped through a sublimator (evaporator) where thecarbon dioxide particles are sublimated at least partly, thereby coolingthe sublimator (evaporator) which may be used for the cooling of air,e.g. for freezing and storing of food at so low temperatures as fromabout -60° C. to about -80° C.

The effluent from the evaporator/sublimator containing terpene, carbondioxide vapor and remaining carbon dioxide particles, is separated suchthat the carbon dioxide vapor may be sucked into a compressor andconverted to liquid state in a condenser. The liquid carbon dioxide maythereafter be returned into the mixing tank for a new cooling cycle.

SUMMARY OF THE INVENTION

A main object of the present invention is to improve the operationalreliability of the prior art sublimation system.

An other object of the present invention is to increase the efficiencyof such an improved system.

Further objects and advantages of the present invention will be obviousfrom the following description.

According to the invention a refrigeration system is provided whichcomprises

a mixing tank for a slurry of solid, sublimatable particles in a liquid,said mixing tank having first and second inlets and an outlet;

a sublimator having an inlet, an outlet and several internal pathsconnecting the inlet and the outlet;

a first conduit connecting the outlet of the mixing tank to the inlet ofthe sublimator for the supply of said slurry of solid particles in aliquid to the sublimator;

a separator having an inlet and top and bottom outlets;

a second conduit connecting the outlet of the sublimator to the inlet ofthe separator for returning sublimated particles and the slurry of stillsolid particles in the liquid from the sublimator to the separator, thebottom outlet of the separator being connected to the first inlet of themixing tank for returning the slurry of still solid particles in theliquid to the mixing tank, the top outlet of the separator ejecting thesublimated particles;

means connected to the second inlet of the mixing tank to make up thesublimated solid particles ejected from the top outlet of the separator;and

further comprising means for continuously agitating the slurryinthemixing tank.

By continuously agitating the slurry in the mixing tank, a primarysource of clogging of the solid particles is eliminated.

Although the refrigeration system according to the invention can bedriven by gravity, a pump may be inserted into the first conduit forpumping the slurry from the mixing tank to and through the sublimator.

Preferably, the refrigeration system according to the invention also hasno descending parts in the conduit leading from the pump to thesublimator and no descending paths within the sublimator, therebyeliminating clogging of the solid particles from the outlet of the pumpto the outlet of the sublimator.

In a preferred embodiment, the mixing tank has an inlet connected to asource of a stirring medium which preferably is the slurry itselfobtained from the outlet of the pump in the first conduit.

Preferably, the solid particles consist of carbon dioxide and the liquidis d'limonene. This leads to such possible improvements as a smallerfreezer, a faster freezing a higher freezing capacity and also avariable capacity based on sublimator temperature. Also, the lowtemperature of the sublimator/evaporator reduces the frost depositionthereon and lengthens the time interval between defrosting stops of thesystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a preferred embodiment of arefrigeration system according to the present invention.

FIGS. 2-4 illustrates alternative embodiments of the separator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the system shown in the drawings, carbon dioxide is used as coolingmedium in combination with d'limonene as transport medium. However, itshould be noted that the invention is not limited to these substancesbut could as well use other substances with corresponding properties,i.e. a first constituent being immiscible in a second liquid constituentand being capable of sublimating at temperatures appropriate forfreezing, the second constituent still being liquid at the sublimatingtemperatures of the first constituent.

Referring to FIG. 1, a refrigeration system according to the inventioncomprises a mixing and separating tank 1, a pump 2, asublimator/evaporator coil 3, a conduit 4 connecting a bottom outlet 5of the mixing and separating tank 1 with an inlet 6 of the evaporatorcoil via an inlet and an outlet of the pump 2, and a conduit 7connecting an outlet 8 of the sublimator/evaporator coil 3 with an inlet9 of the mixing and separating tank 1.

A compressor 10 has an inlet 11 connected to a top outlet 12 of themixing and separating tank 1 by means of a conduit 13 and an outlet 14connected to a condenser 15 followed by a receiver 16 which in its turnis connected to a bottom inlet 17 of the mixing and separating tank 1via a valve 18 and by means of a conduit 19.

A heat exchanger 20 is inserted in the conduits 13 and 19 such thatcarbon dioxide vapor flowing through the conduit 13 is heated by theliquid carbon dioxide flowing through the conduit 19. As a consequenceof this superheating of the carbon dioxide vapor, the cost of thecompressor 10 may be reduced substantially.

A supply tank 21 is optionally provided for additional supply of liquidcarbon dioxide on demand via a valve 22 into the conduit 19 and throughthe valve 18 to the bottom inlet 17 of the mixing and separating tank 1.Preferably, the supply of liquid carbon dioxide from the supply tank 21only takes place when the the demand of liquid carbon dioxide is abovethe capacity of the compressor, i.e for top loads on thesublimator/evaporator 3.

A conduit 23 connects the outlet of the pump 2 with a bottom inlet 24 ofthe mixing and separating tank 1 via a valve 25.

The refrigeration system described operates as follows. The mixing andseparating tank 1 contains a slurry of solid carbon dioxide particles ina liquid of d'limonene. The pump 2 sucks this slurry from the tank 1 viathe bottom outlet 5 thereof such that the slurry is forced through theconduit 4 to the inlet 6 of the sublimator/evaporator coil 3, throughthis coil 3 to its outlet 8 and via the conduit 7 back to the inlet 9 ofthe mixing and separating tank 1.

A fan blows air through the evaporator coil 3 such that the solid carbondioxide particles entrained by the d'limonene transport fluid sublimateto carbon dioxide vapor during the passage through thesublimator/evaporator coil 3. According to the invention, theconcentration of solid carbon dioxide in the refrigerant, i.e. theslurry of carbon dioxide particles in the d'limonene transport liquid,entering the evaporator coil 3 should be so high that an excess amountof solid carbon dioxide particles still is present in the effluent fromthe outlet 8 of the sublimator/evaporator coil 3. This excess of solidcarbon dioxide particles ensures an efficient cooling of the wholeinternal area of the sublimator/evaporator coil 3.

By making the paths of the refrigerant from the pump 2 to and throughthe evaporator ascending or at least horisontal, i.e. not descending,according to the present invention, the risk of clogging of the solidcarbon dioxide particles is completely eliminated. Thus, the flow of theslurry should always be upward or at least level from the pump 1 to andthrough the sublimator/evaporator 3.

Further, the risk of accumulation of the solid carbon dioxide particlesat the bottom of the mixing and separating tank 1 is eliminated by thecontinuous agitation produced by that part of the slurry which is fedback to the bottom inlet 24 of the mixing and separating tank 1 by thepump 2 via the conduit 23 and the valve 25.

It should be understood, that the agitation could be realized by otherstirring media as well as by other means, such as mechanical means.

The refrigerant returning into the mixing and separating tank 1 from thesublimator/evaporator coil 3 via the conduit 7 and the inlet 9 consistsof liquid d'limonene, solid carbon dioxide particles and carbon dioxidevapor. Preferably, the inlet 9 is positioned above the surface of theslurry in the mixing and separating tank 1 and directed tangentiallysuch that the carbon dioxide vapor follows an upwardly directed pathtowards the top otlet 12 of the mixing and separating tank 1, while thed'limonene liquid and the solid carbon dioxide particles are injectedinto the slurry in the same tank 1.

The compressor 10 sucks the substantially dry carbon dioxide vapor intoits inlet 11 via the conduit 13 from the top outlet 12 of the mixing andseparating tank 1, the carbon dioxide vapor being superheated in theheat exchanger 20, i.e. to a temperature of at leat -50° C., in order toenable the compressor 10 to operate safely for a reasonable time. Also,this superheating makes it possible to use a compressor of lesssophisticated design and thus of less cost. The liquid carbon dioxidefed from the receiver 16 via the conduit 19 and the valve 18 through theinlet 17 could be used as a heating medium in the heat exchanger 20.Alternatively, ammonia used in a prestage for cooling the condenser 15may be used as the heating medium in the heat exchanger 20.

The inlet 17 of the mixing and separating tank 1 is preferably a bottominlet in order that the liquid carbon dioxide when injected therethroughand transformed into solid carbon dioxide and carbon dioxide vaporshould act as a vigorous stirring medium in the slurry of solid carbondioxide particles in liquid d'limonene, However, since the injection ofliquid carbon dioxide may be discontinuous, that injection might takeplace at another position and the stirring effect thereof replaced byanother stirring mechanism, such as described above. It should be notedthat a substantial part of the liquid carbon dioxide is transformed intoflash gas when introduced into the mixing and separating tank 1. Thisflash gas raises the pressure at the outlet 12 of the mixing andseparating tank 1. In order not to overload the compressor 10, a valve26 may be connected to the outlet 12 so as to vent carbon dioxide vaporfrom the mixing and separating tank 1 to the atmosphere when thepressure thereof exceeds a predetermined limit value.

Further, the momentary value of the vapor pressure inside the mixing andseparating tank 1 could be used for regulating the valve 18 such thatthe pressure does not exceed the predetermined limit. Thus, the value ofthe pressure within the mixing and separating tank 1 could be used asinput value to a PID regulator controlling the opening of the valve 18via an electric motor.

The refrigerant in the mixing and separating tank 1 should have such acarbon dioxide concentration that the refrigerant pumped into thesublimator/evaporator 3 is overfed with carbon dioxide and thereby coolsall the internal surfaces of the sublimator efficiently.

The concentration of solid carbon dioxide in the slurry fed into thesublimator/evaporator 3 may be controlled by the use of a light sensingdevice 27 to genrate a signal indicative of said concentration, e.g.indirectly by representing the turbidity of the slurry, for regulatingthe valve 18 by means of an appropriate control system 28 and thus theflow rate of liquid carbon dioxide supplied to the mixing tank 1.

Alternatively, the temperature difference and/or the pressure differencebetween the inlet 6 and the outlet 8 of the sublimator/evaporator 3 maybe used as a controlling input to the control system 28 in order toregulate the flow rate of liquid carbon dioxide supplied to the mixingtank 1. This alternative is shown schematically in FIG. 1 by boxeslabeled "S" (for temperature or pressure sensor) which are coupled tothe system 28.

In FIG. 1, the mixing and separating tank 1 contains the separator as anupper part thereof, the lower part being used for mixing the solidcarbon dioxide particles and the liquid brine for the transport of thoseparticles. However, the separating and mixing functions are preferablyperformed in substantially separate vessels, as illustrated in FIGS.2-4.

In FIG. 2, a mixing and separating tank 1' has an inner funnel-shapedpartition 29 forming the bottom of an upper separating section 30 andhaving a bottom outlet 31 submerged into the slurry in a lower mixingsection 32. More than half of the liquid carbon dioxide introducedthrough the inlet 17 being vaporized, the partition 29 comprises atangential vent 33 in order to equalize the pressures in the lowersection 32 and the upper section 30. The flash gas thus generated in thelower section 32 passes through the vent 33 having the form of a nozzlesuch that the vapor is accelerated tangentially within the funnel-shapedupper section 30. Thus, the slurry in the lower section 32 is agitatedby the liquid carbon dioxide from the inlet 17 and the resulting carbondioxide vapor is centrifugally separated from any entrained droplets ofbrine before returning to the compressor 10 via the top outlet 12.

As illustrated in FIG. 3, the direct vent 33 into the upper section 30can be replaced by a pipe 34 having a pressure regulator 35 such that apredetermined pressure difference may exist between the lower section 32and the upper section 30 acting to pump the slurry out through theoutlet 5 towards the pump 2. Of course, the pressure difference must belower than the pressure from the column of slurry coming out of thefunnel-shaped bottom part of the upper section 30.

Still another embodiment is illustrated in FIG. 4, wherein a firstseparate vessel 36 is used for the separation of the refrigerantreturned from the sublimator/evaporator 3 via the inlet 9 and a secondseparat vessel 37 is used for the mixing of the solid carbon dioxideparticles and the low temperature brine. In FIG. 4, the pipe 34 and thepressure regulator 35 connect the first and second separate vessels 36and 37 for the same purpose as in the embodiment shown in FIG. 3.

It is to be understood that modifications, alterations and changes canbe made in the refrigeration system without departing from the scope ofthe invention as claimed herein. Thus, it is intended that the abovedescription and the accompanying drawings shall be interpreted asillustrative and not in a limiting sense.

What we claimed is:
 1. A refrigeration system comprising:a tank having abottom outlet (5) and a top outlet (12), said tank further comprising anupper separator chamber having therein the top outlet and including aseparator inlet (9), and a lower mixing chamber for a slurry of solid,sublimatable particles in a liquid, said mixing chamber having thereinthe bottom outlet and including a first mixing inlet (17); a sublimatorhaving a sublimator inlet (6), a sublimator outlet (8), and a pluralityof internal paths connecting the sublimator inlet and the sublimatoroutlet; a first conduit (4) connecting the bottom outlet (5) to thesublimator inlet (6) for supply of said slurry of solid particles in aliquid to the sublimator; a second conduit (7) connecting the outlet (8)of the sublimator to the separator inlet (9) for returning gas composedof sublimated particles and the slurry of still solid particles in theliquid from the sublimator to the separator chamber, the top outlet (12)ejecting the gas composed of sublimated particles; a sublimated solidparticle supplier (10, 11, 14-16, 20) connected to the first mixingchamber inlet (17) to make up the sublimated solid particles ejected asgas from the top outlet; and further comprising an agitator (23-25) forcontinuously agitating the slurry in the mixing chamber.
 2. Arefrigeration system comprising:a separator having therein a top outlet(12) and including a separator inlet (9); a mixing tank for a slurry ofsolid, sublimatable particles in a liquid, said mixing tank beingdisposed below the separator and including a bottom outlet (5), a firstmixing tank inlet (17), and a second mixing tank inlet (31) disposed inan upper portion thereof and communicating with a lower portion of theseparator; a sublimator (3) having a sublimator inlet (6), a sublimatoroutlet (8), and a plurality of internal paths connecting the sublimatorinlet and the sublimator outlet; a first conduit (4) connecting thebottom outlet (5) to the sublimator inlet (6) for supply of said slurryof solid particles in a liquid to the sublimator; a second conduit (7)connecting the outlet (8) of the sublimator to the separator inlet (9)for returning gas composed of sublimated particles and the slurry ofstill solid particles in the liquid from the sublimator to theseparator, the top outlet (12) ejecting the gas composed of sublimatedparticles; a sublimated solid particle supplier (10, 11, 14-16, 20)connected to the first mixing chamber inlet (17) to make up thesublimated solid particles ejected as gas from the top outlet; andfurther comprising an agitator (23-25) for continuously agitating theslurry in the mixing chamber.
 3. A refrigeration system as claimed inclaim 2, wherein the mixing tank has a further inlet below the level ofthe slurry and connected to a source of a stirring medium.
 4. Arefrigeration system as claimed in claim 3, comprising a pump in thefirst conduit for pumping the slurry from the mixing tank to and throughthe sublimator, said pump forming said source and having an outletconnected to said further inlet of the mixing tank.
 5. A refrigerationsystem as claimed in claim 4, wherein the first conduit has nodescending part between the pump and the inlet of the sublimator.
 6. Arefrigeration system as claimed in claim 2, wherein the solid particlesconsist of carbon dioxide and the liquid is a low temperature brine. 7.A refrigeration system as claimed in claim 6, wherein the liquid isd'limonene.
 8. A refrigeration system as claimed in claim 6, wherein theflow rate of carbon dioxide into the mixing tank is controlled inresponse to the difference between the temperature of the slurry at theinlet of the sublimator and the temperature of the slurry at the outletof the sublimator.
 9. A refrigeration system as claimed in claim 8,wherein the flow rate of carbon dioxide into the mixing tank also iscontrolled in response to the difference between pressure at the inletof the sublimator and the pressure at the outlet of the sublimator. 10.A refrigeration system as claimed in claim 6, wherein the flow rate ofcarbon dioxide into the mixing tank is controlled in response to thedifference between pressure at the inlet of the sublimator and thepressure at the outlet of the sublimator.
 11. A refrigeration system asclaimed in claim 6, further comprising a pump in the first conduit forpumping the slurry from the mixing tank to and through the sublimator,and a compressor having an inlet connected to the top outlet of theseparator and an outlet connected to the second inlet of the mixingtank.
 12. A refrigeration system as claimed in claim 11, furthercomprising a sensor of the concentration of solid carbon dioxide at theoutlet of the pump for controlling the flow rate of liquid carbondioxide supplied to the mixing tank.
 13. A refrigeration system asclaimed in claim 2, further comprising a compressor having an inletconnected to the top outlet of the separator and an outlet connected tothe second inlet of the mixing tank.
 14. A refrigeration system asclaimed in claim 2, further comprising a supply tank of liquid carbondioxide connected to the second inlet of the mixing tank.
 15. Arefrigeration system as claimed in claim 14, further comprising a valvecontrolling the flow rate of liquid carbon dioxide from the supply tankin response to a demand of liquid carbon dioxide above the capacity ofthe compressor.
 16. A refrigeration system as claimed in claim 15,further comprising a sensor of the concentration of solid carbon dioxideat the outlet of the pump for controlling the flow rate of liquid carbondioxide supplied to the mixing tank.
 17. A refrigeration system asclaimed in claim 2, wherein the slurry contains solid carbon dioxide inexcess such that also the effluent from the sublimator contains solidcarbon dioxide particles.
 18. A refrigeration system as claimed in claim2, wherein the separator is contained in the mixing tank.
 19. Arefrigeration system as claimed in claim 18, wherein the bottom outletof the separator is submerged in the slurry in the mixing tank.
 20. Arefrigeration system as claimed in claim 19, wherein the separator has afunnel-shaped bottom part.
 21. A refrigeration system as claimed inclaim 20, wherein the funnel-shaped bottom part forms a partitionbetween the separator and the mixing tank.
 22. A refrigeration system asclaimed in claim 18, wherein the separator is formed by an upper part ofthe mixing tank.
 23. A refrigeration system as claimed in claim 2,wherein the separator is in gas communication with an upper part of themixing tank.